1. Field of the Invention
The present invention relates to a printing apparatus, conveyance apparatus, and conveyance control method. Particularly, the present invention relates to a printing apparatus and a feed-conveyance control method which supply a printing medium such as a printing paper sheet and print on the printing medium by causing a printhead to discharge ink.
2. Description of the Related Art
The following method is conventionally known to prevent a printing medium such as a printing paper sheet from being diagonally conveyed in a printing apparatus upon feed. A conveyance roller is stopped or rotated in a direction reverse to the conveyance direction of a printing medium. In this state, the printing medium is conveyed by a feed roller until its tip(leading edge) abuts against the conveyance roller. In this case, generally, after a printing medium position detection sensor provided immediately before the conveyance roller detects the tip position of the printing medium, the printing medium is conveyed by the feed roller by the distance from the sensor to the conveyance roller until its tip abuts against the conveyance roller.
Japanese Patent Laid-Open No. 2002-347296 discloses an arrangement that counts the number of times the duty value of a driving pulse signal by PWM (Pulse Width Modulation) has reached the maximum value. If the count value has reached a predetermined value, the duty value (to be described later) of PWM is switched to “0” to interrupt voltage application to a DC motor. This arrangement stops power supply to the DC motor to prevent heat generation in it if the DC motor is locked.
The technique described in this prior art presumes to determine abutting of the tip(leading edge) of a printing medium depending on whether or not the PWM duty value exceeds a predetermined threshold value. In this case, if feed conditions such as the type of printing medium, variations in conveyance mechanism load, and motor performance change, the timing when the duty value exceeds the threshold value also changes. For this reason, it is not able to properly prevent a printing medium from being diagonally conveyed.
FIG. 8 is a graph for explaining control for preventing diagonal conveyance by using a conventional technique.
In this conventional technique, when an output PWM value exceeds a predetermined threshold value, conveyance is stopped by setting the value (PWM along the ordinate in FIG. 8) to “0”. The abutting detection timing changes as indicated by ΔX in FIG. 8 depending on the output PWM value before abutting against a conveyance roller. This may make it impossible to prevent a printing medium from being diagonally conveyed or wrinkle a printing medium due to overabutting.